The invention is based on a hydraulic dual-circuit brake system with an anti-skid system and traction control for motor vehicles.
In a known dual-circuit brake system of this type (German Patent 38 16 073, now U.S. Pat. No. 4,900,102, the wheel brake cylinders of the driven wheels are assigned to one brake circuit, and the wheel brake cylinders of the nondriven wheels are assigned to the other. The control valve assemblies for feeding a wheel slip-dependent brake pressure into the wheel brake cylinders are embodied by 3/3-way magnet valves, which are connected in pairs per brake circuit, each via a respective connecting line, to the associated brake circuit outlets of the master brake cylinder. Also connected to the same connecting lines are the outlet valves of the pump elements of the return pump, of which only one pump element is operative in one brake circuit. The electromagnetic valve unit, operative only during traction control, is embodied by a 3/2-way magnet valve, which is incorporated into the connecting line between one brake circuit outlet from the master brake cylinder and the two control valve assemblies assigned to the wheel brake cylinders of the driven wheels. In its unexcited basic position, the 3/2-way magnet valve makes this connecting line open, and in its working position, which is always brought about during traction control, it disconnects the master brake cylinder from the aforementioned control valve assemblies and connects these assemblies to the brake fluid tank for return of the fluid to the tank. A pressure limiting valve is provided in the segment of the line between the 3/2-way magnet valve and the brake fluid tank. During traction control (also known as ASR), the precharge pump, embodied as a low-pressure pump, feeds the pump element of the return pump, which generates a high brake pressure that is present at the inlet to the control valve assemblies for the wheel brake cylinders of the driven wheels. Excess brake fluid is returned to the brake fluid tank via the 3/2-way magnet valve and the pressure limiting valve. The brake pressure generated by the pump element of the return pump is fed into the wheel brake cylinder of the driven wheel exhibiting slip at a given time, and this wheel is thereby braked.
The known dual-circuit brake system is designed solely for rear-wheel drive vehicles, which have a so-called front-axle/rear-axle or front/rear brake circuit distribution.